The use of implantable cardioverter defibrillator (ICD) as a primary prevention strategy for sudden cardiac death (SCD) has dramatically reduced mortality in selected patient populations. Randomized controlled trials have shown that in heart failure (HF) patients with reduced left ventricular ejection fraction (LVEF), prophylactic ICD implantation improves survival compared with standard medical therapy by detection and treatment of malignant ventricular arrhythmias. However because ICD implantation poses significant risk and is costly, the use and outcomes associated with this therapy outside trial participants and in high-risk subgroups is an important focus by patients, clinicians and policy-makers alike. Chronic kidney disease (CKD), defined as estimated glomerular filtration rate (eGFR) <60 ml/min/1.73 m2, affects approximately one-third of the 5.7 million adults with heart failure (HF) in the U.S. Among HF patients, reduced kidney function has been shown to be the most powerful predictor of SCD. Despite a high burden of and high risk associated with reduced kidney function among HF patients, most previous trials and studies of primary prevention ICD implantation have excluded or have been underpowered to study patients with reduced kidney function. Thus, whether the risks and benefits of primary prevention ICD implantation differs among patients with reduced kidney function compared with those with preserved kidney function remains unclear, but has critical therapeutic implications for this high-risk, growing patient population. We plan to merge two unique cohorts within the ongoing NHLBI-funded Cardiovascular Research Network (CVRN), a national, large, exceptionally-characterized patient registry of 15 health systems to address these gaps in knowledge: (1) a cohort of patients with HF and LVEF<50% (CVRN-HF-REF) and (2) the Longitudinal Study of Implantable Cardioverter-Defibrillators (LSICD), a cohort of patients undergoing primary prevention ICD placement. Merging these two overlapping cohorts, we will create the CVRN-RISK (CVRN-Risks with ICD placement and outcomeS in Kidney disease) cohort to accomplish the following specific aims. In Aim 1, we will evaluate the association between level of kidney function and likelihood of receiving an ICD for primary prevention of sudden cardiac death among patients with HF with reduced LVEF. In Aim 2, we will determine the association of pre-implantation kidney function with post-implantation infections/hematomas and inappropriate shocks among patients receiving a primary prevention ICD. In Aim 3, we will evaluate if kidney function modifies the comparative effectiveness of receipt versus no receipt of a primary prevention ICD on HF hospitalizations and death among HF patients with reduced LVEF. Finally in Aim 4, we will develop a decision and cost-effectiveness model to evaluate the net benefit versus harm associated with primary prevention ICD placement at different levels of kidney function among patients with HF with reduced LVEF.